1. Field of the Invention
The present invention relates to an objective lens for a pickup optical system for optically reading information recorded on a recording medium such as a compact disk (CD) or a digital video disk (DVD).
2. Related Background Art
As a recording medium, a disk-like compact disk (CD) or a digital video disk (DVD) having the same diameter as that of a CD is known. A CD or DVD is formed by laminating a pair of transparent disks whose thicknesses are equal, and a recording surface on which information is recorded is arranged between the disks. A CD and DVD have different thicknesses. For this reason, even when a DVD is set in a CD player, information recorded on a DVD cannot be read because the position of the recording surface is different from that of the CD.
Under these circumstances, a player capable of reading information from both a CD and DVD has been developed. Such a player normally has an accommodation region for accommodating the recording medium such as a CD and DVD, and a read unit which moves along the radial direction of the CD or DVD is arranged at a position opposing the accommodation region. The read unit incorporates a pickup optical system. Light from a light source is focused on the recording surface of the CD or DVD through the objective lens of the pickup optical system.
Such an objective lens is used for a so-called twin-lens pickup optical system. This pickup optical system has two objective lenses with different focal lengths. Light is focused on the recording surface of the CD or DVD by switching these objective lenses for the CD or DVD.